Process of dehydrating aqueous acetic acid with carbon tetrachloride



I June 1932- H. T. CLARKE ET AL 5 1,

PROCESS OF DEHYDRATING AQUEOUS ACETI C ACID WITH CARBON TETRACHLORIDEFiled Feb. 15, 1930 3 Sheets-Sheet l Hats 2? Clarke & Donaldl'YOihmenJune 7, 1932. H. T. CLARKE ET AL 1,861,841

S PROCESS OF DEHYDRATING AQUEOUS ACEfIIC ACID WITH CARBON TETRAGH LORIDEFiled Feb. 15, 1930 s Sheets-Sheet 2 mml w 'June 7, 1932. H. T. CLARKEETAL 1,361,841

PROCESS OF DEHYDRATING AQUEOUS ACETIC ACIDWITH CARBON TETRACHLORIDE ssheets-sheet 3 Filed Feb. 15, 1930 Acetic Actd m'fler.

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gwuentow EOihmen Hans Q'Zarke &Donald Patented June 7 1932 UNITED STATESPATENT OFFICE HANS T. CLARKE, OF NEW YORK, AND DONALD F. OTHMER, OFROCHESTER,.-NEW YORK,

ASSIGNORS TO EASTMAN KODAK COMPANY, OF ROCHESTER, NEW YORK, A GOR-PORATION OF NEW YORK PROCESS OF DEHYDRATING AQUEOUS ACETIC ACID WITHCARBON TETRAGHLORIDE Application filed February 15, 1930. Serial No.428,642.

This invention relates to processes for removing water from aqueousacetic acid and particularly such processes in which a third componentis added to the aqueous acid solu- 5 tion to remove the water therefromby distilling the constant boiling mixture which is formed.

There have been numerous methods suggested, both for laboratory andcommercial purposes. for the removal of water from aqueous acetic acidsolutions. For example, th aqueous acetic acid solution is subjected toextraction by a solvent having greater solvent action for the aceticacid than for the Water and subsequently distilling from the separatedextraction liquid the acetic acid and the extracting medium. Low boilingsolvents such as ether or ethyl acetate have been suggested by Brewsterfor such an extraction 0 process, while high boiling solvent materialssuch as wood tar oils, phenols and similar extracting media have beensuggested by Suida. Another general method for conducting theconcentration of acetic acid from 5 aqueous solutions consists in addingto the aqueous acid solution a third component which is only slightlysoluble in water but infinitely soluble in acetic acid and distillingfrom this mixture an azeotropic mixture of 0 water. acetic acid and thethird component. The etticiency of this latter type of process isgoverned largely by the proportion of water removed relative to theacetic acid that comes over during this part of the distillationprocess, the best third component being of course one which would carryover no acetic acid. but a fairly high proportion of water. An object ofthe present invention is to provide a. process that will be simple.inexpensive, and yet applicable to acid solutions of any strength, highor low. Another object is to provide a process which will be applicablenot only to relatively pure aqueous solutions of acetic acid. but alsoto the prod action of concentrated and glacial acetic acid rom crudeaqueous acetic acids, such as those containing pyroligneous liquor fromwood distillation. A further object is to provide a process in whichdistillation of the aqueous acid is carried out with the aid of anauxiliary organic liquid which combines the essential properties of suchan auxiliary in a way not heretofore known. Other objects willhereinafter appear.

This application is in part a continuation of our application Serial No.232,916,- filed Nov. 12, 1927, defining the dehydration of aqueousacetic acid by the use of ethylene dichloride and azeotropicdistillation.

In the accompanying drawings:

Figure 1 is a diagrammatic side elevation of one form of apparatus inwhich the process may be carried out, the parts being relativelyexaggerated for the sake of clearness;

Figure 2 is a similar View of another apparatus in which the process maybe carried out;

Figure 3 is a similar View of the preferred apparatus for carrying outour process.

Figures 4 and 5 are charts showing the.

water removing possibilities from aqueous acetic acids of difierentconcentrations if distilled alone and when distilled with an auxiliaryliquid suchas carbon tetrachloride.

There are numerous technical processes, in which acetic acid is employedand from which the acetic acid somewhat diluted is recovered, in whichit is necessary to reconcentrate the acetic acid for further use.

Such a process is employed in the preparation of cellulose acetate. Inthe acetylation of cellulose the acetic anhydride used is partlyconverted to acetic acid and this, together with the acetic acid addedto the acetylating mixture as a diluent, constitutes the greaterproportion of the cost of manufacturing this ester. It is thereforenecessary, in order to economically produce cellulose acetate, that theacetic acid be recovered and preferably reconcentrated to asconcentrated a form as possible. Up to the present time a largeproportion of the acetic acid produced is prepared from the distillationof wood. The acid in this form, contaminated with empyreumaticimpurities, together with Wood tars and a large proportion of water,mustlikewise be concentrated to put it in condition for use in the art.

Our process of removing the water from aqueous acetic acid is primarilyone of distillation in contradistinction to an extraction process. Wehave found that acetic acid can readily be concentrated from its aqueoussolution if a third auxiliary liquid be added to the acetic acid duringor prior to distillation. We have found likewise that an auxiliaryliquid having many advantages for this purpose is carbon tetrachloride.The vapor obtained from the distillation of an aqueous solution ofacetic acid to which carbon tetrachloride has been added will consist,after suitable rectification, in a binary azeotropic mixture of waterand carbon tetrachloride and substantially no acetic acid.

By continuing such a rectification process, the preferred method ofwhich is to be hereinafter described, the water is gradually removedfrom the acetic acid until there is produced an anhydrous acid whichmay, however, contain carbon tetrachloride depending on the amountoriginally added.

The process is preferably carried out in fractionating columns of knowntypes, the distilled water and carbon tetrachloride being condensed andallowed to settle into two layers and the carbon tetrachloride whichforms the lower layer being returned to the upper part of the column. Aminute proportion of acetic acid may be carried over with the othervapors in which case the acid Will be distributed between the water andcarbon tetrachloride layers, although it will be understood that thisproportion of acetic acid is really negligible and can be disregarded inmost instances. In the preferred embodiment of our process, the carbontetrachloride passes through a cycle without serious loss and can beused over and over again.

Carbon tetrachloride satisfies many of the requirements of an auxiliaryliquid for use in distilling water from aqueous acetic acid in that itdoes not readily react with the acetic acid nor is it decom used whenboiled with that acid. It is likewise available in considerablequantities and at not too great a cost. It boils at a temperature belowthat of acetic acid and it can be readily and efiiciently separated fromthat acid by distillation and rectification. It forms a binaryazeotropic mixture with water, which mixture contains an appreciableproportion of water, but does not form such a constant boiling mixturewith acetic acid nor does it form a ternary azeotropic mixture withwater and acetic acid. It is likewise almost completely insoluble inWater. The requirement that the concentration of acetic acid in thewatery layer of the dis- .tillate be lower than that'of the aqueousacetic acid which is being concentrated is likewise satisfied by the useof carbon tetrachloride as an auxiliary liquid. The latent heat of thesolvent in the composition of the azeotropic mixture with water vapor issuch that unit amount of water (in the azeotropic mixture) is likewiselow.

Carbon tetrachloride is, therefore, useful in that it fulfills very wellthe above requirements for a good auxiliary liquid for the concentrationof aqueous acetic acid solutions. Its boiling point at atmosphericpressure is approximately 76 C. (This is about 41 C. lower than that ofaetic acid). Thus the solution containing acetic acid and carbontetrachloride can be readily separated into its two constituents bydistillation. Carbon tetrachloride forms an azeotropic mixture withwater vapor which boils under atmospheric pressure, at approximately 69C. It is soluble in water to only a very slight extent less than .1 percent by weight at 20 C.

Using the proper rectifying column there is substantially no acid (lessthan per cent) in the watery layer of the distillate. Since distillationand rectifying processes will not concentrate aqueous acids weaker thanthe watery layer of the distillate, and since our process in properapparatus causes the presence of virtually no acid in such layer, ourprocess can concentrate aqueous aetic acids of any strengthfrom lessthan 1 per cent to over 99 per cent.

Beference to Figures 4 and 5 will show still further the specialadvantages of carbon tetrachloride as an auxiliary liquid. In eachfigure the abscissae are the percentages of water relative to the weightof acid in the liquid being distilled without any rectification and theordinates are the percentage of water relative to the acid in the waterylayer of the distillate. In other Words, such percentage of water wasfirst determined in the liquid to be distilled. The latter was thendistilled long enough to et a test distillate. The test distillate was alowed to settle to form a watery layer and the percentage of the waterdetermined in this layer.

In each figure there is, for convenience, a straight line at 45 whichindicates equality in the percentages. Points above this 45 lineindicate that the water can be distilled off to concentrate the originalacid.

Figure 4 shows the conditions when a mixture of acetic acid and wateralone is distilled. It will be seen that, no matter what the strength ofthe aqueous acid is, the percentage of water in the correspondingdistillate is only slightly greater, the curve being only a little abovethe straight line. This shows one reason Why it is uneconomical todistill and rectify the aqueous acetic acid. alone in order to effectconcentration thereof.

Figure 5 gives the curve for a mixture of acetic acid, water and carbontetrachloride. This curve 'was obtained by simple distillation. Let usassume, for example, that we are distilling an acetic acid solutioncontaining 50 per cent water. In the Figure 4 it will be seen that a 5per cent acetic acid water liquid will give a vapor containingapproximately 62 per cent water. In Figure 5, on the other hand whendistilled in the presence of anexcess of carbon tetrachloride it will benoted that a 50 per cent liquid acetic acid will give a vapor which oncondensation yields an aqueous,phasecontaining slightly more than 72 percent water. The efliciency of rectifying and concentrating acetic acidmay be regarded as being in approximately the same proportion as theincrease in per cent of water in the watery layer of the dis-, tillatesobtained in the two cases. The advantage of our process is, therefore,apparent.

Referring to the accompanying drawings, Figure 1 shows diagrammaticallyone of the simplest ways in which our method may be carried out. Thedilute aqueous acetic acid enters by pipe 1, controlled by valve'2 intothe acid heater 3, the latter being heated by a steam jacket or steamcoil in the conventional way through the pipe 4.

The vapors from the heated dilute acid enter the fractionating column 5,which may be of any of the well known types, such as that employingplates with bubble cap construction, and in this fractionating columnmeet a downward current or wash of carbon tetrachloride coming fromthenozzle 6 sent through pipe 7 from supply 8, valve 9 controlling theflow. An azeotropic mixture of watervapor and carbon tetrachloride isobtained at the top of a sufliciently eflicient column and passesthrough the pipe 90 into condenser 10, is cooled in any convenient Wayby cooling fluid supplied through pipe 11, and the condensate thusformed, passes through pipe 12 into the settling vessel 13. Here thedistillate separates, by gravity, into two layers, the lower one beingcarbon tetrachloride which can be drawn off through pipe 14' which iscontrolled by valve 15. This carbon tetrachloride may then bere-introduced into supply 8. The upper layer is composed chiefly ofwater, but may contain small amounts of acetic acid and carbontetrachloride. It may be drawn off through pipe 16, controlled by valve17. If the amount of carbon tetrachloride in it is of any consequence,this may be recovered by a flashing and condensing process-that is, onein which a current of steam is blown through the water and the carbontetrachloride thus evolved is condensed. This operation may be conductedin an auxiliary column if desired. The liquid which finally results invessel 3, will be completely dehydrated.

Since an excess of carbon tetrachloride is used, say more thanapproximately 10 times the weight of water to be removed, the liquid invessel 3 will finally become a dehydrated mixture of carbontetrachloride and glacial acetic acid. This can be withdrawn throughpipe 300 controlled by valve 301 and the two ingredients separated byfractional distillation in any usual apparatus, this being easy becauseof the wide difference in the boiling points of the two ingredients andthe fact that no constant boiling mixture of acetic acid and carbontetrachloride is formed.

Figure 2 shows a further modification in which the major part of thecarbon tetrachloride passes rapidly through a definite cycle.

The dilute aqueous acetic acid enters through pipe 18 controlled byvalve 19 into the heating apparatus 20, the latter being supplied withthe necessary heating fluids for its jacket or coils through pipe 21.The ascending vapors in fractionat-ing column 22, of known type such asthat provided with refractive packing, meet a descending spray or streamof carbon tetrachloride coming from the nozzle 01' opening 23. .Theazeotropic mixture of vapors, including water and carbon tetrachloridewith possibly a trace of acetic acid passes through pipe 24 into thecondenser 25, which receives its supply of cooling fluid through pipe26. From the condenser it flows through pipe 27 into the settling vessel28. The upper layer of water is drawn off through pipe 29, controlled byvalve 30, and the lower layer of carbon tetrachloride is conducted backthrough nozzle 23 from the settling vessel 28 through pipe 31 controlledby valve 32. Also there is an auxiliary supply 35 of carbontetrachloride from which the latter may be conducted to nozzle 23through pipe 36 controlled by valve 37. This supply is merely tocompensate for any losses which take place during the regular cycle ofthe carbon tetrachloride through elements 24, 25, 27, 28, 31 and 23. Thewater, which is drawn oil, can be run to waste or any carbontetrachloride contained therein flashed out and condensed, as explainedabove in connection with Figure 1. The dehydrated mixture of carbontetrachloride and glacial acetic acid is drawn oil' from the heater 20through the pipe 200, controlled. by valve 201 and then fractionated toseparate the ingredients, as is also above described.

Figure 3 shows diagrammatically the preferred apparatus for carrying outour invention. A supply tank of aqueous acetic acid 38 is connected by apipe 39, controlled by valve 40, with .an intermediate ortion (say abouttwo-thirds of the way up of a fractionating column 41 of the usual typesuch as that provided with plates and bubble cap construction.

At the top of this column a downward current or spray of carbontetrachloride enters from the nozzle or opening 42, which is connectedwith a horizontal transfer pipe 43. The base of the column is providedwith the customary heating vessel 44, the heating fluid for which entersthrough the pipe 45. The azeotropic mixture of the vapors of water andcarbon tetrachloride leaves the top of the column, passing through ipe46 into condenser 47, the coohng fluid oFwhich circulates through pipe48. The condensate which collects in the condenser 47 flows down intothe settling chamber 49, the latter being provided with a partition orbafiie 50 extendmg downwardly to within a short distance from the bottomof the vessel. The pipe 51 connected to the condenser 47 likewiseextends well down into the vessel 49. Then the condensate reaches vessel49, it separates into two layers, the water layer being uppermost andconfined to one side of the parti-' tion 50, as shown in the drawings.From the vessel 49 the watery layer passes 011 through the pipe 52 tofurther treating apparatus which will be described hereinafter. Thelower layer of carbon tetrachloride containing more traces of aceticacid and water passes beneath the partition 50 and through pipe 53 topipe 43 and then downwardly through the nozzle 42 into the fractionatingcolumn 41. Thus the bulk of the carbon tetrachloride passes through 'acycle from nozzle 42 to column 41 and then through the followingparts,46, 47, 51, 49, 53 and 42.

But as there is an excess of carbon tetrachloride used,namely more thanabout 1-0 parts by weight of the carbon tetrachloride for each part byweight of water to be eliminated, this means that some carbontetrachloride will collect with the dehydrated or glacial acetic'acid inthe heated vessel 44'. This dehydrated mixture is then conducted throughpipe 54 controlled by the valve 551 to an intermediate portion of anauxiliary fractioning column 55 of one of the known types.

The heating for this column 55 is done in the chamber 56, the heatingfluid for which circulates through the pipe 57. Vapors of carbontetrachloride pass from the top of the column through pipe 58 intocondenser 59, the cooling fluid of which circulates through pipe 60.This condensed carbon tetrachloride can be passed partly through pipe 61controlled by valve 62 intp pipe 43 and nozzle 42 of the mainfractionating column, and partly through pipe 63 controlled by valve 64and nozzle or opening 65 at the top of the column 55, there to act as arefluxing liquid for wash in the column.

The glacial acetic acid in vessel 56 is conducted through pipe 66 to asimple still 67, the heating fluid for which circulates through pipe 68.Thevapors of acetic acid pass over through pipe 69 into the finalcondenser 70 from which the glacial acetic acid can be drawn off andstored. The distillation in still 67 is not indispensable, but it ispreferable to remove some small amount of coloring or polymerized bodiesthat may be formed during the other operations.

Referring to the upper left hand part of 55 Figure 3, the watery layerfrom the settling vessel 49 passes through pipe 52 to the nozzle oropening 71 at the top of the fractionating column 72, the latterbeing ofany of the usual types. Steam is blown into the column through pipe 73and hot water passes to waste through pipe preferably tov a heatinterchanger not shown. Passage of the steam up through column 72flashes 011 the small amount of carbon tetrachloride which is dissolvedin the watery material descending from the nozzle 71. The vapors ofcarbon tetrachloride pass throu h pipes 74 into condenser 75, thecooling lates through pipe 76. The carbon tetrachloride from 75 passesthrough pipe 51 to the bottom or lower layer of the settling vessel 49and thence returns to the normal circulatory path of. the carbontetrachloride through pipes 53 and 43 and nozzle 42.

It will be understood that in all forms of I apparatus, the customaryprecautions for preventing heat losses by suitable insulatlon areobserved and the parts which contact with the acid are mad-e ofmaterials resistant to corrosive action of acetic acid customarilyemployed for that purpose. Furthermore the process is preferablyoperated under atmospheric pressure conditions, although it can beconducted under either super-atmospheric pressure or sub-atmosphericpressure. lVhen we refer herein to the boiling points of the ingredientsand mixtures such, for instance, as the boiling point of water, we referto those under the particular pressure conditions that areemployed,normally atmospheric.

From a consideration of the above disclosure, it will be evident thatcarbon tetrachloride may be used for efi'ecting the concentration ofacetic acid in quantities below that designated, in such cases it beingevident that a complete dyhydration of the acetic acid will not beefiected; in some cases a complete dehydration may not be required.

What we claim as our invention and deslrc to be secured by LettersPatent of the United States is:

1. In the process of removing water from aqueous acetic acid, the stepsof mixing therewith carbon tetrachloride and distilling water and carbontetrachloride from the mixture.

2. In the process of removing water from aqueous acetic acid, the stepsof mixing therewith carbon tetrachloride and distilling water and carbontetrachloride from the mixture at a temperature below the boiling pointof water.

3. In the process of removing water from aqueous acetic acid, the stepsof mixing therewith more than approximately 10 parts by Weight of carbontetrachloride for each part of water to be removed and distilling waterand carbon tetrachloride from said mixture.

fluid of which circu- 4. In the process of removing water from aqueousacetic acid, the steps of mixing therewith more than about 10 parts byweight of carbon tetracholoride for each part of water to be removed anddistilling off the water with part of the carbon tetrachloride at atemperature below the boiling point of water and separating thedehydrated acetic acid from the remaining carbon tetrachloride.

5. In the process of removing water from aqueous acetic acid, the stepsof mixing therewith carbon tetrachloride distilling the mixture at atemperature below the boiling point of water to evolve a constantboiling mixture of water and carbon tetrachloride, condensing the lattermixture, allowing it to settle into layers and returning the lower layerto the original mixture undergoing distillation.

6. In the process of removing water from aqueous acetic acid, the stepsof supplying said acid to a distilling column and during distillationtherein adding carbon tetrachlo-.

ride at the upper part of said column, and removing from the top of thecolumn a constant boiling mixture of water and carbon tetrachloride at atemperaturebelow the boiling point of water.

7 In the process of removing water from aqueous acetic acid, the stepsof supplying said acids to a distillin column, during the distillationtherein adding carbon tetra chloride at the upper part of said column,and removing from the top of "the column a constant boiling mixture ofwater and carbon tetrachloride at a temperature below the boiling pointof water, the weight of trichlorethylene added to said column being morethan about 10 times the weight of water to be removed.

8. In the process of removing water from aqueous acetic acid, the stepsof supplying said acid to an intermediate part of the distilling column,during the distillation therein adding carbon tetrachloride at the upperpart of said column, removing from the top of the column a constantboiling mixture of water and carbon tetrachloride, condensing the lattermixture and whatever acid comes over with it, allowing it to settle intotwo layers,returningthelowerlayer to the upper partof the column andwithdrawing the mixture of dehydrated acetic acid andcarbontetrachloride from the lower part of the column, and separatingthe former from the latter, the total weight of carbon tetrachlorideadded to the upper part of the column per unit time being more thanabout 10 times the weight of water distilled from the top of the columnper unit times, and the temperature at which said constant boilingmixture is removed being below about C.

Signed at Rochester, New York this 11th day of February 1930.

HANS T. CLARKE.

DONALD F. OTHMER.

